The present invention relates to a liquid dispensing apparatus adapted for accurately metering and discharging a predetermined quantity of viscous liquid such as resin material, conductive adhesive, cream solder, UV resin, silver paste and the other paste like material used in coating, dotting, fixing and injecting processes.
A liquid dispensing apparatus discussed for example in Japanese patent application laid open 63-97259. This paste dispensing apparatus comprises a syringe containing a paste or the like, an air supply for supplying air under a positive pressure to the syringe, a dispensing solenoid valve arranged between the syringe and the air supply. The solenoid valve is connected to the syringe by means of a first pipe line and to the air supply by means of a second pipe line for communicating the syringe with the air supply when the paste is dispensed from the syringe. A first pressure sensor measure a pressure in the first pipe line and a second pressure sensor measures a pressure in the second pipe line. A controller having inputs for signals of measured pressure is connected to the first and second pressure sensor. The controller also has a voltage signal output for actuating the dispensing solenoid valve to connect the first and second pipe lines to each other at the start of dispensation. The controller controls a duration of the output voltage signal until the pressure measured by the first pressure sensor reaches a preset pressure after the first and second pipe lines are communicated to each other.
The aforementioned paste dispensing apparatus further comprises an air suction device connected to the dispensing solenoid valve by a third pipe line to connect the first and third pipe lines while the first and second pipe lines are not connected to each other. The air suction device supplies air under a negative pressure to the first pipe line to prevent the paste from dripping from the syringe after completion of dispensing.
The controller outputs a voltage signal that actuates the dispensing solenoid valve to supply a positive air pressure from the air supply to the syringe thereby dispensing the paste from the syringe. A variation in the pressure measured by the first pressure sensor in relation to the amount of paste remaining in the syringe is estimated by the controller so that the amount of the paste dispensed by the syringe is constant in spite of a variation in the amount of the paste remaining in the syringe.
The aforementioned paste dispensing apparatus can be controlled to dispense a predetermined amount of paste from the syringe when the air pressure in the syringe reaches a preset pressure P as shown by a curve q in FIG. 5. The aforementioned dispensing apparatus however cannot be controlled to accurately dispense the paste from the syringe when the air pressure in the syringe does not reach the preset pressure P as shown by a curve in FIG. 5 because of change or variation of syringe inner volume, pipe line inner diameter, or compressed pressure.
The present invention solves the foregoing problems and provides an improved liquid dispensing apparatus capable of dispensing a constant amount of liquid in spite of variations in the curve of air pressure in the syringe. The present invention also quickly and effectively prevents liquid from dripping out of the syringe between repeated dispensations of the liquid.
A liquid dispensing apparatus according to the present invention comprises a syringe containing liquid, an air supply for supplying air under a positive pressure to the syringe, a dispensing solenoid valve arranged between the syringe and the air supply, a first pipe line connecting the syringe to the dispensing solenoid valve, a second pipe line connecting the dispensing solenoid valve to the air supply, a first pressure sensor for measuring the internal pressure in the first pipe line, a second pressure sensor for measuring the internal pressure in the second pipe line, an accumulator connected to the second pipe line, and a controller having a shifting output signal to the dispensing solenoid valve. The controller controls an output time of the shifting output signal in accordance with variation of an integral value of pressure measured by the first pressure sensor integrated with a preset output time of the shifting output signal after the first and second pipe lines are connected by the dispensing solenoid valve.
The liquid dispensing apparatus according to the present invention may further comprise a suction solenoid valve connected to the dispensing solenoid valve by means of a third pipe line, an air suction source connected to the suction solenoid valve by means of a fourth pipe line, a vent solenoid valve connected to the third pipe line, and an accumulator connected to the fourth pipe line wherein each of the solenoid valves are connected to the controller.
In operation, the controller of the liquid dispensing apparatus according to the present invention outputs a shift signal of a voltage signal to the dispensing solenoid valve to supply air under a positive pressure from the air supply to the syringe for a duration corresponding to a quantity of liquid remaining in the syringe, to thereby accurately dispense liquid from the syringe. In order to supply air under positive pressure corresponding to the quantity of liquid in the syringe, an amount of liquid remaining in the syringe is determined by an integrated value of pressure measured by the first pressure sensor during a predetermined voltage signal output duration thereby permitting a determination of a required dispensing time. Thus, the controller outputs a voltage signal to the dispensing solenoid valve for the determined time to supply air under a positive pressure into the syringe.
According to the present invention, variation in the rising pressure curve caused by variation of quantity of liquid remaining in the syringe as measured by the first pressure sensor is detected as a variation in the integrated value during a set time. Accordingly, it is possible to prevent a problem occurring in the prior art which results from detection of the variation in the rising curve as a variation in the time required for reaching the set pressure.
According to another aspect of the present invention, the atmospheric pressure or a negative pressure from air suction devices is applied to the syringe simultaneously with the termination of the supply of air under a positive pressure to the syringe, so that the dripping of liquid from the syringe is effectively prevented.
According to another aspect of the present invention, the pressure measured at the beginning of the supply of air under pressure to the syringe until the internal pressure in the syringe reaches a preset pressure can be made even by means of an accumulator so that a duration of the output voltage signal from the controller can be controlled with high accuracy without any influence from variations in the positive pressure supplied from the air supply source. Specifically, the accumulator connected to the fourth pipe line can reduce the internal pressure in the syringe to a predetermined negative pressure in short time and prevent a variation in the internal pressure in the syringe which may be caused by pulsation in the air suction source. Furthermore, the vent opening solenoid valve connected to the third pipe line can open to reduce the internal pressure in the syringe to the atmospheric pressure in a very short time with less pipe friction and instantly stop the discharge of liquid out of the syringe to thereby greatly shorten the dispensing cycle time.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following Detailed Description of the Invention and the accompanying Drawings described below .